1. Field of the Invention
The present invention relates in general to an electric-component mounting system, and a method of detecting a positioning error in the system, more particularly to techniques that facilitate the detection of the positioning errors.
2. Discussion of Related Art
Representative examples of an electric-component mounting system arranged to mount electric components (typically, electronic components) on a circuit substrate such as a printed-wiring board include a system having a plurality of mounting heads, and a head-turning device which carries the mounting heads and which is rotated to turn the mounting heads about a common axis of turning, for mounting the electric components on the circuit substrate. In operation of this electric-component mounting system, each of the mounting heads that are turned about the common axis of turning is sequentially moved to and stopped at a plurality of working positions or stations, which include a component-receiving position at which each mounting head receives an electric component from a component-supplying device, and a component-mounting position at which the mounting head transfers the electric component onto a circuit substrate held by a substrate-holding device. The substrate-holding device is arranged to hold the circuit substrate, and is moved by a substrate-positioning device such that a component-mounting spot on the circuit substrate is located right below the mounting head located at the component-mounting position, so that the electric component is mounted at the component-mounting spot on the printed-wiring board or other circuit substrate.
In a known electric-component mounting system of the type described above, the circuit substrate is provided with fiducial marks whose images are taken by an image-taking device, so that a positioning error of the circuit substrate is detected on the basis of image data representative of the taken images of the fiducial marks. The positioning error of each component-mounting spot is obtained on the basis of the detected positioning error of the circuit substrate, and the position of the circuit substrate is adjusted to eliminate the positioning error, so that each electric component can be mounted at the component-mounting spot on the circuit substrate, with a high degree of positioning accuracy. Namely, movement data used by the substrate-positioning device to position the circuit substrate upon mounting of each electric component are adjusted or compensated to eliminate the positioning error.
The actual mounting position of the electric component as mounted on the circuit substrate may more or less deviate from the nominal mounting position, even where the positioning error of the component-mounting spot on the circuit substrate is eliminated upon mounting of the electric component on the circuit substrate. For example, this deviation may arise from a positioning error of the image-taking device, which is included in the positioning error of the component-mounting spot detected on the basis of the images of the fiducial marks taken by the image-taking device. Accordingly, there arises an error of relative positioning between the mounting head and the component-mounting spot on the circuit substrate so that there arises a positioning error of the electric component as mounted on the circuit substrate. Similarly, a positioning error of the electric component as mounted on the circuit substrate may arise from a positioning error of the mounting head, which causes an error of relative positioning between the mounting head and the component-mounting spot on the circuit substrate.
In view of the drawback described above, the image-taking device and the mounting head at the component-mounting position are conventionally pre-positioned relative to a zero point of the substrate-positioning device, by an operator of the system, using a suitable measuring instrument or jig. This manual pre-positioning procedure permits accurate relative positioning between the image-taking device and the substrate-holding device when the circuit substrate is moved to a position at which the images of the fiducial marks are taken by the image-taking device, so that the positioning error of the circuit substrate as held by the substrate-holding device can be detected with a higher degree of accuracy. Accordingly, the mounting head can be accurately aligned with the nominal component-mounting spot on the circuit substrate, by adjusting the movement data used for the substrate-positioning device to position the circuit substrate, so as to eliminate the detected positioning error of the component-mounting spot, when the electric component is mounted on the circuit substrate by the mounting head. Thus, the manual pre-positioning is effective to improve the positioning accuracy of each electric component as mounted on the circuit substrate.
However, the conventionally practiced manual pre-positioning using the measuring jig is cumbersome and time-consuming, inevitably delaying starting of a production run of the electric-component mounting system if the manual pre-positioning is effected immediately before the mounting of the electric components. Further, it is difficult to perform the manual pre-positioning during the production run of the system, so that the movement data for the substrate-positioning device cannot be compensated for the positioning error of the circuit substrate or the component-mountings spots which may arise from thermal expansion of various parts of the system in operation, and which may deteriorate the positioning accuracy of the electric components as mounted on the circuit substrate.
The problem described above is also encountered in an electric-component mounting system of a type in which a mounting head is moved by a head-positioning device in a plane parallel to the component-mounting surface of a circuit substrate as held by the substrate-holding device, so that each electric component is moved with the mounting head to a position right above the corresponding component-mounting spot on the circuit substrate, at which the electric component is mounted by the mounting head.